This invention relates to the art of extrusion machinery and more particularly to the type of extrusion machinery known as a single roller die extruder.
Extruders are normally used for the production of continuous strips having substantially constant cross-section. The material to be formed or "extrudate" is fed into the extruder, brought to a fluid or semi-fluid condition by heat, mechanical agitation or a combination of heat and mechanical agitation and forced out through an orifice having a shape generally corresponding to the cross-section desired in the finished strip.
For example, in the tire industry, an extruder is used to produce tire sidewall strips; raw rubber is fed into the extruder, fluidized by mechanical action and heat in the extruder and forced out through the orifice. Single roller die extruders are particularly suited to the production of tire sidewall strips because of their ability to produce a strip having thin edges without introducing defects or blemishes into the strip.
Single roller die extruders include a generally cylindrical roller and a stationary die head. The die head has a surface which confronts an arcuate portion of the cylindrical surface of the roller and cooperates with the roller to define an extrudate chamber. Means are provided for introducing an extrudate into this extrudate chamber. The roller is rotated about its axis to force the extrudate in a downstream direction towards a extrudate shaping orifice defined by a downstream portion of the confronting surface of the die head and the surface of the roller itself.
The direction downstream, as used in this disclosure, should be understood to mean the direction towards which the surface of the roller moves in its rotation, while the term upstream should be understood to mean the opposite direction. The terms axial and axially will also be used in this disclosure and should be understood as referring to the axis of rotation of the roller.
As can be appreciated, the thickness of the strip issuing from a single roller die extruder is controlled, at least in part, by the clearance between the stationary die head and the roller at the extrudate shaping orifice.
In the single roller die extruders of the prior art, such as those set forth in U.S. Pat. Nos. 3,871,810 and 3,869,304, the roller is positioned by the frame of the apparatus in proximity to the stationary die head so that the clearance between the surface of the die head which forms part of extrudate shaping orifice and the roller will yield the desired strip thickness and shape.
Although such apparatus is capable of producing satisfactory strips, it is difficult to accurately control the thickness of the finished strip produced by such apparatus. The material in the extrudate chamber between the roller and the die head is under substantial pressure. Although the exact pressure in the chamber is unknown, it is believed to be about 250 pounds per square inch at the point immediately upstream from the orifice. In an extruder of typical size, this extrudate pressure acting on the roller and die head creates a force on the order of 3000 pounds tending to deflect the apparatus frame and move the roller away from the die head. This movement of the roller makes it difficult to accurately maintain the clearance between the roller and the die head, and therefore makes it difficult to control the thickness of the finished strip issuing from the extruder.